As one of the conventional means for coupling an optical device such as a semiconductor laser to an optical fiber or for coupling optical fibers to each other at a high coupling efficiency, an optical fiber with a lens is used.
The optical fiber with a lens can be a single-mode optical fiber or polarization maintaining optical fiber respectively directly worked at a tip of the optical fiber to form a spherical portion at the tip, or a multi-mode optical fiber or coreless fiber fusion-coupled to a tip of a single-mode optical fiber or polarization maintaining optical fiber and worked at the tip of the multi-mode optical fiber or coreless fiber to form a spherical portion at the tip, or the like.
For example, U.S. Pat. No. 5,774,607 describes an optical fiber with a lens, obtained by working a tip of a coreless fiber with an isotropic refractive index to have a spherical portion at the tip, and coupling the coreless fiber to a single-mode optical fiber using a multi-mode optical fiber. The length of the multi-mode optical fiber is set at the ¼ length of the meandering period of the sinusoidally propagating light or at an odd-number multiple of the ¼ length.
In this case, the meandering period of light in the multi-mode optical fiber is obtained by approximate calculation based on the values of core diameter, specific refractive index and the like on the assumption that the refractive index distribution is a perfect square distribution. One-fourth of the meandering period is called the ¼ length. The optical fiber with a lens obtained by using a multi-mode optical fiber with its length set at the ¼ length is disclosed in U.S. Pat. No. 6,130,972 and many other documents.
U.S. Pat. No. 5,774,607 also describes a single-mode optical fiber directly worked at a tip to have a spherical portion formed at the tip, as the prior art.
With regard to a single-mode optical fiber, for example in a typical case of using light with a wavelength of 1.55 μm, the spot size is as small as about 10.4 μm. So, in the case where the optical fiber is directly worked at a tip to form a spherical portion at the tip, such conditions as concentricity and shape accuracy during working for forming a spherical portion are severe, and it is inevitable that the focal distance becomes short, and that the working distance is as very short as, for example, about 5 μm. So, when it is coupled to an optical device or another optical fiber, it is very likely that they are caused to collide with each other in aligning work, etc.
On the other hand, in the case where a multi-mode optical fiber is coupled to a tip of a single-mode optical fiber and is worked to form a spherical portion at the tip of the multi-mode optical fiber, the meandering period of light is calculated approximately for setting the ¼ length on the assumption that the square distribution of refractive index continues infinitely toward outside. On the contrary, in an actual multi-mode optical fiber, the square distribution of refractive index extends finitely only up to the radius of the core, and a clad with a uniform refractive index exists outside the core. So, there occurs an approximation error compared with an actual case, and it is difficult to obtain the highest coupling efficiency.
As described above, in the prior art in which a multi-mode optical fiber is coupled to a tip of a single-mode optical fiber and is worked to form a spherical portion at the tip of the multi-mode optical fiber, it is the most important matter to set the length of the multi-mode optical fiber at the ¼ length in which the beam undulating and propagating in the optical fiber expands to the maximum extent, or at an odd-number multiple of the ¼ length. On the contrary, in this invention, the following matters are most important.
Without arriving at the ¼ length from the beginning, the inventors found that the following two matters are important when an optical fiber is coupled to an optical device such as a semiconductor laser or to another optical fiber using an optical fiber with a lens.
(1) The beam intensity distribution at the focal position (beam waist) of the beam condensed by an optical fiber with a lens is as close to a Gaussian distribution as possible.
(2) The beam spot sizes of the optical articles to be coupled to each other should agree with each other.
For forming an optical fiber with a lens satisfying the above-mentioned two important matters, the inventors found that it is necessary that the mode of the beam propagating in a multi-mode optical fiber coupled to a single-mode optical fiber or polarization maintaining optical fiber consists of the fundamental mode only and does not contain the excitation of higher modes or the leak mode propagating in the core-clad interface. They further found it necessary to have a forming method that allows the focal distance to be changed easily and freely, in order to make the spot size variable.
This invention has been completed based on the above-mentioned findings. An object of this invention is to solve the above-mentioned problems by fusion-coupling a multi-mode optical fiber to a tip of a single-mode optical fiber or polarization maintaining optical fiber, and working the tip of the multi-mode optical fiber to form a spherical portion at the tip. Another object of this invention is to reduce the aberration for enhancing the coupling efficiency by adequately setting the radius of curvature of the spherical portion formed at the tip of the multi-mode optical fiber and the length of the multi-mode optical fiber.